Batteries and capacitors represent two most important systems for energy storage, with applications in electronics, electric vehicles, telephone communication systems, power supplies, and many other applications.
A battery is used to store electricity by converting electric energy into chemical energy during charging and converting chemical energy back into electric energy during discharging. The energy is stored through chemical reactions, which are often associated with the change of oxidation states of active metal species (faradic reactions). Electro-active materials are often the active component in batteries since they can provide redox reactions for energy storage. Because of the high theoretical energy storage capacities, metals have been pursued as the negative electrode materials for high energy batteries. Batteries based on metals, however, generally have poor cycling stabilities during the charge/discharge process. The instability normally comes from the irreversible metal dissolution/deposition process. Metals may dissolve into metal ions during the discharge process in those batteries. For example, in a zinc-nickel oxide battery, metallic zinc dissolves as zinc ions during discharging and deposit back as metallic zinc from zinc ions during charging. This dissolution/deposition process is not repeatable as zinc tends to form dendrites in solution instead of depositing back onto the current collector as the original metallic film. Similar problems have been encountered in lithium batteries, where the metallic lithium dissolves as lithium ions during discharging and the lithium ions deposit as metallic lithium during charging. Capacitive materials will be more stable if they can maintain their solid morphology during the charge/discharge process. For example, in lithium ion batteries, metals such as tin may form alloys with lithium ions during charging and the alloys release lithium ions back into the electrolyte during discharging. During the charge/discharge cycling process, the non-lithium metals experience volume expansion/contraction because the insertion/extraction of lithium ions. However, since the solid morphologies are maintained for these metals, the cycling stability has been greatly improved for tin-based lithium ion batteries as compared to lithium batteries. The typical charge/discharge cycling number is only at most tens of cycles for metallic lithium-based batteries, while the typical cycling number for lithium ion batteries with tin negative electrodes can be hundreds of cycles. These non-dissolving electro-active materials, however, may still have limited charge/discharge cycling stability mainly because of the volume change associated with the faradic reactions. The lithium ion insertion and extraction processes will cause the volume of the non-dissolving electro-active material to expand and to contract, which will disrupt the original compact structure of the electrode film resulting in the loss of electric connections among the electro-active particles, which in turn results in the fading of capacity during the charge/discharge cycling. Unlike the dissolution/deposition process, instability from the volume expansion/contraction process can be limited by controlling the material architecture in the electrode. For example, the stability can be improved greatly by coating the electro-active material onto a stable carbonaceous material, so that the electric connections to the electro-active material can be maintained during the volume expansion/contraction process.
As another type of energy storage devices, electrochemical capacitors store electric energy mainly through the highly reversible electric static interactions (double-layer adsorption/desorption or non-faradic reactions). Since there is negligible physical state change of the electrode material during the charge/discharge process, an electrochemical capacitor can have excellent cycling stability up to 20,000,000 cycles. Electrochemical capacitors, however, are limited in energy densities. In a capacitor, the amount of charge that can be stored is directly proportional to the available electrode/electrolyte interfaces for ions adsorption/desorption. The maximum energy density that can be stored therefore is limited by the surface area of the electrode material. The energy density can be improved by using an asymmetric structure, where one electrode consists of an electro-inert porous carbonaceous material and the other electrode consists of an electro-active material. The incorporation of an electro-active material can greatly increase the energy density of the device by using less total electrode materials since the electro-active material can have at least several times larger capacity than an electro-inert material. A capacitor's capacitance can be calculated as 1/(mTCT)=1/(mnCn)+1/(mpCp), where CT is the device's specific capacitance, Cn is the specific capacitance for the negative electrode, Cp is the specific capacitance for the positive electrode, mT is the total weight of the two electrode materials, mn is the weight for negative electrode weight, and mp is the weight for positive electrode weight. For a symmetric double-layer capacitor, Cn equals Cp. To maximize the value of CT, it is necessary to make mnCn and mpCp to be the same value resulting in a mass ratio of 1 of mn/mp. CT therefore is ¼ of Cn or Cp. In an asymmetric capacitor, if Cp (electro-active material) is far larger than Cn (porous carbon), mp could be much smaller than mn. mT therefore will be close to the weight of mn yielding a much larger value of CT. J. P. Zheng calculated the theoretical limitation of energy densities for the two systems. The maximum energy density is 7.16 Wh/kg for an activated carbon/activated carbon symmetric capacitor, while the value reaches 50.35 Wh/kg for an activated carbon/Ni(OH)2 asymmetric capacitor with aqueous electrolyte. Regardless of the advantage of electro-active materials, charge capacities of these materials, however, are still limited (for example theoretical capacity for NiOOH is 292 mAh/g). Higher energy density can be expected if the positive electrode can store much more energy so that the electrode weight can be further reduced.
As a hybrid system of batteries and fuel cells, metal-air batteries have been a focus as the energy storage devices with high energy densities. In this system, a pure metal such as lithium and zinc is used as fuel to generate electrons as the negative electrode. Oxygen from air is used as the oxidant at the positive electrode side. During discharging, the metal is oxidized at the negative electrode, while oxygen is reduced by a catalyst at the positive electrode (air electrode). During charging, the oxidized metal ions are reduced into metal particles, while oxygen is generated by using an oxygen evolution catalyst at the positive electrode. Since oxygen can be fed from air, the theoretical energy capacity at the air electrode is unlimited large. The theoretical energy density of this system is determined by the energy capacity of the metal and the operating voltage window.
This system, however, is generally limited by its poor charge/discharge cycling stability, which is mainly caused by the instability of the negative electrode during cycling. The prior art air batteries have been limited into either using metals as negative electrode materials in aqueous alkaline electrolyte or metallic lithium as the negative electrode material in organic electrolyte except in very few cases where silicon and metal hydrides were used as the negative electrode materials in aqueous alkaline electrolyte and carbonaceous materials and metals were used as the negative intercalation materials in organic electrolyte. The cause of the instability varies depending on the electrolyte system and the negative electrode material. For zinc-air, sodium-air, magnesium-air, and lithium-air batteries, the instability is mainly because of the metal dissolution/deposition process during the charge/discharge cycling process. In U.S. Patent Application, Pub. No. 2006/0257744 A1, Burchardt disclosed the formulation of a zinc electrode for electrochemically rechargeable zinc-air alkaline batteries. The stability is still limited to tens of cycles because of the stability limitation in zinc electrode. In U.S. Patent Application, Pub. No 2007/0117007 A1, rechargeable lithium-air batteries were fabricated by depositing a lithium-ion conductive solid coating on metallic lithium to limit the corrosion of lithium by moisture and electrolyte. This coating, however, is not expected to solve the stability issues because of the dramatic volume change of the lithium film during the lithium dissolution/deposition process. In Patent Publication, Pub. No. WO/2010100636, Yair disclosed the use of a silicon negative electrode in an alkaline system as a primary air battery. The formed silicate ions are almost impossible to be reduced back into silicon. Osada et al. disclosed the use of metal hydrides as the negative electrode materials in the 218th Electrochemical Society Meeting and a promising cycling stability was reported. The cycling stability of metal hydrides, however, is limited to at most 1500 cycles as in nickel metal hydride alkaline batteries. In U.S. Patent Application, Pub. No. 2004/0241537 A1, Okuyama et al. revealed the fabrication of an air battery with carbonaceous materials and metals as the negative electrode material in an organic electrolyte. These negative materials stored energy through a lithium ion intercalation process (faradic reactions) as being described in the patent. For carbonaceous materials and metals to be used to as intercalation materials, the negative electrode has to be charged/discharged to very low voltage potentials (generally <0.3 V vs. Li/Li+), which may limit the long-term stability and cause safety issues. Carbonaceous materials generally act as electro-inert materials in aqueous solution, but they can become electro-active when they are charged/discharged to very low potentials in organic electrolyte (generally <0.3 V vs. Li/Li+). An electro-active carbonaceous material will experience volume expansion/contraction as other negative electrode materials, which limits its cycling stability from tens to a few thousands of cycles. In comparison, an electro-inert carbonaceous material may be cycled for millions of cycles. Moreover, electro-active carbonaceous materials may be limited in energy capacities. The theoretical energy capacity for graphite is 374 mAh/g. In comparison, the theoretical energy capacity for silicon is 4200 mAh/g. On the other hand, non-lithium metals can form alloys with lithium. These metals will experience dramatic volume expansion/contraction during charge/discharge cycling. For example, the volume expansion for tin is 676% when it is fully charged in a lithium ion battery. A battery with a pure metal negative electrode will have limited cycling stability because of the volume expansion/contraction. In this sense, prior art negative materials for non-aqueous air batteries are limited either in specific capacity (carbons, 374 mAh/g or 834 mAh/cm3 for graphite) or in cycling stability (lithium and metals). It would be necessary to develop a negative material that provides both high capacity and good cycling stability for air batteries.
The instability of the metal electrode has limited metal-air batteries to mainly primary (non-rechargeable) batteries unless the metal is mechanically refueled, which is similar to the operation concept with a liquid or gaseous fuel. Besides being limited in the negative electrode materials, the prior art metal-air batteries have several other limitations. The prior art metal-air batteries are limited in using air electrode as the positive electrode, while the possibility of using air electrode as the negative electrode has not been disclosed. The prior art aqueous metal-air batteries are limited in using a highly corrosive alkaline solution as the electrolyte, while the possibility of using a mild neutral solution or an acidic solution has not been disclosed.